Machine vision sensors and identification (ID) readers are used for reading printed and direct part mark (DPM) codes for manufacturers implementing part traceability programs in the automotive, aerospace, electronics, healthcare, defense, and other industries and for other applications such as quality control during production of a product, supply chain processes and end use applications. Exemplary ID codes include 1-D barcodes and 2-D codes Code 128, Code 39, UPC, EAN, interleaved 2 of 5, and QR Code. The rapid adoption of DPM codes and other identification for part traceability has manufacturers demanding a rugged, self-contained reader that performs consistently on all types of codes and parts. In order for an optical detection system such as an ID or symbology reader to reliably read everything from the most challenging DPM codes to relatively simple printed barcodes, proper illumination of the object on which the ID code is marked or printed is required.
In the case of identification symbol detection using an image sensor or camera, the type of symbology detected affects illumination requirements. For example, a UPC barcode can be printed on a label or packaging and can present a relatively high contrast and corresponding readability to an optical ID reader, typically having black bars on a white background. Where barcodes and other viewed subjects are printed on a flat surface with contrasting ink or paint, a diffuse, high-angle “bright field” illumination may best highlight these features for the sensor. By high-angle it is meant, generally, that light strikes the object on which a mark is applied nearly perpendicularly (normal) or at an angle that is typically no less than about 45 degrees from perpendicular (normal) to the surface of the item being scanned. Such illumination is subject to substantial reflection back toward the sensor. By way of example, barcodes that require mainly bright field illumination may be present on a printed label adhered to an item or container, or on a printed field in a relatively smooth area of item or container.
In contrast, an advancing and growing field for smart cameras or imaging systems is direct part mark (DPM) identification where, as the label implies, marks are directly applied to parts (e.g. via etching, dot peening, etc.). An exemplary widely used DPM code is the well known Data Matrix code.
When a symbol or mark is etched or peened onto a surface of a part or a component and the surface of the part or component is rough or irregular, high-angle bright field illumination may not be appropriate. To this end, irregular surfaces of mark features tend to scatter as much light back to the reader as the surface to which the mark is applied, resulting in indistinguishable features in the image.
Low angle, “dark field” illumination has been shown to be suitable for certain direct part marking applications. Dark field illumination includes low-angle illumination that strikes a marked surface at a low angle (e.g., at an angle between 45 degrees and 90 degrees from an axis perpendicular to the surface). Using such low angle dark field illumination, two-dimensional surface texture is contrasted more effectively (with indents appearing as bright spots and the surroundings as shadow) for improved image detection and recognition. Further, certain reading applications may yield higher successful read rates when a combination of bright field and dark field illumination is used.
ID readers are known which provide dark field illumination by illuminating a light pipe with light emitting diodes (LEDs) where an angled surface at the end of the light pipe reflects the LED light to provide low-angle illumination of a marked surface. Bright field illumination has been provided by other sets of LEDs and diffusers. The light from the bright field LEDs are incident on a reflector which is directed toward the object, and the reflected light then passes through a diffuser before it is incident on the object.
Known ID reader lighting assemblies have several shortcomings. First, intensity differences between LEDs have been known to create non-uniformities in the illumination. Second, known systems require multiple LED sets and/or additional optics, such as light pipes, reflectors and diffusers. Third, in the case of the bright field illumination, the diffuser absorbs some of the light, thereby reducing the brightness of the light applied to the marked surface. Fourth, in the case of the bright field illumination, the LEDs are off the optical axis of the reflector, which can also create non-uniformities in the illumination.